Failed to receive on two Rx channels with B210

[sada45]

Issue Description

Hi, I'm trying to receive IQ signal from two Rx channels with B210 and Python APIs. It works fine while using one Rx channel. However, if the st_args.channels = [0, 1], it returns the error Invalid recv stream command - stream now on multiple channels in a single streamer will fail to time align.
Then, I tried the solution provided in the #325 to set the stream_cmd.stream_now = False. But it turns out that the collected sample are all zeros.

Setup Details

UHD: 4.3.0.0, Python: 3.8.10, OS: Ubuntu 20.04, USRP: B210

Expected Behavior

Actual Behaviour

While using two channels and stream_cmd.stream_now = True

Traceback (most recent call last):
  File "/home/sada45/BLong/USRP/test.py", line 35, in <module>
    streamer.issue_stream_cmd(stream_cmd)
RuntimeError: RuntimeError: Invalid recv stream command - stream now on multiple channels in a single streamer will fail to time align.

If the stream_cmd.stream_now = False, the error is gone, but received samples are all 0:

Steps to reproduce the problem

My code to collect the signal samples is:

b210 = uhd.usrp.MultiUSRP("type=b200")
b210.set_rx_rate(config.sample_rate, 0)
b210.set_rx_freq(uhd.libpyuhd.types.tune_request(get_freq_with_chan_num(config.chan)), 0)
b210.set_rx_gain(config.gain, 0)
st_args = uhd.usrp.StreamArgs("fc32", "sc16")
st_args.channels = [0,1]
metadata = uhd.types.RXMetadata()
streamer = b210.get_rx_stream(st_args)
recv_buffer = np.zeros((2, 1000), dtype=np.complex64)
stream_cmd = uhd.types.StreamCMD(uhd.types.StreamMode.start_cont)
stream_cmd.stream_now = False
streamer.issue_stream_cmd(stream_cmd)
streamer.recv(recv_buffer, metadata)
print(recv_buffer[0, 0:10])
print(recv_buffer[1, 0:10])

stream_cmd = uhd.types.StreamCMD(uhd.types.StreamMode.stop_cont)
streamer.issue_stream_cmd(stream_cmd)

Additional Information

[mbr0wn]

2 channel RX definitely works. Bump your gain and you should be fine. Sorry for leaving this open for so long.